Such a method is known from U.S. Pat. No. 5,270,552. The method is used for the preparation of minute samples to be observed and/or analysed in a particle-optical apparatus such as a Transmission Electron Microscope (TEM).
As is well-known a sample to be studied in a TEM needs to be extremely thin. For organic samples, as used in biology, a thickness of up to 1 μm may be used, but for high-resolution images of biological material a thickness of less than 100 nm is preferred. For samples comprising atoms with heavier nuclei, such as semiconductor samples, a thickness of less than 100 nm, preferably less than 50 nm is commonly used.
In the known method a small sample is severed from a work piece such as a semiconductor wafer. To that end the sample is cut free from the work piece by e.g. milling with an ion beam. Before the sample is completely cut free, it is attached to a manipulator tip using e.g. IBID (Ion Beam Induced Deposition). The sample is then freed completely, and may be transported by the manipulator for further preparation and/or for analysis. As known to the person skilled in the art the further preparation may include attaching the sample to a sample carrier and severing the sample from the manipulator using the ion beam.
As known to the person skilled in the art, when studying a biological sample, such as cells or other biological tissues, in e.g. a TEM, the material must either be embedded in resin or such like, or the sample must be cooled to a cryogenic temperature. The latter is advantageous in that the biological material can be observed in a state closely resembling its natural state in the vacuum of the TEM and is not easily damaged by the electron beam used in the TEM.
The attaching of the manipulator to the sample using IBID, as disclosed in the known method, involves the admittance of a gas to the evacuated sample and irradiating the sample with a focused ion beam. The admitted gas adheres to the sample, and during the irradiation of the sample with the focused ion beam the adhered gas molecules will dissociate. Part of the dissociated molecule will form the bonding material, while the other part of the dissociated molecule will form a volatile gas and leave the surface of the sample.
A problem when using the known method with frozen samples with a cryogenic temperature is that the gasses used for IBID are frozen all over the sample. Not only will this result in a contaminated sample, that is: a sample covered with an unwanted layer, but it will also result in an unreliable bond between sample and manipulator, as at cryogenic temperatures the normally volatile products formed during the dissociation of the adhered molecules will not leave the surface of the sample and may disturb the formation of the bond.